1. Field of the Invention
The invention relates to a reference voltage generating circuit, and more particularly, to a reference voltage generating circuit that quickly switches a capacitor to filter out noise of a predetermined voltage in order to generate a reference voltage.
2. Description of the Prior Art
In the fields of digital-to-analog converters (DAC) and analog-to-digital converters (ADC), the allowable range of amplitudes of an input signal is determined according to the relative voltage levels of the positive and negative reference voltages. The noise standard of each circuit in an ADC or DAC is also determined by the allowable range. In other words, if the relative level of the reference voltage is larger, the design complexity corresponding to the noise is lower.
The relative reference voltage levels generated inside the IC, however, are lower than those generated outside the IC. Therefore, the external voltage source VDD and the ground voltage GND are generally utilized to generate needed reference voltages.
Please refer to FIG. 1, which is a diagram of a conventional reference voltage generating circuit 100. As shown in FIG. 1, the reference voltage generating circuit 100 is an RC circuit, and the external voltage VDD may have noise. After filtering by the RC circuit 100, a clean reference voltage can be generated and provided to a chip.
Unfortunately, a problem is encountered. For example, when the reference voltage generating circuit 100 is utilized in an audio-band analog-to-digital converter (audio-band ADC), for the audio band is about 20 Hz˜20000 Hz, the reference generating circuit 100 should comprise both a large capacitor and resistor to make the corner (−3 db) frequency lower than 20 Hz. Obviously, utilizing a large capacitor and resistor is not a good method of saving costs.
Furthermore, in the application of audio band ICs, power supply rejection ratio (PSRR) is often measured to guarantee that the PSRR can be lower than −60 db in 20 Hz˜20 KHz. If the conventional method is utilized to generate the reference voltage, the PSRR is almost equal to the frequency response of the first order low pass filter. If the corner frequency is at 2 Hz, the PSRR is equal to −20 db at 20 Hz, and −40 db at 200 Hz. Obviously, the PSRR cannot always be lower than −60 db in the frequency band.